Standard deviation and probability (repost)

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Feynboy
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Posting again as attatchment was lost on previous.

b)A nuclear research reactor produces radiation for neutron scattering measurements. A safety procedure shuts the reactor down if a radiation level monitoring detector measures more than 3 counts per minute. In a test, 156 counts are recorded during a random 24 hour interval. What is the corresponding average countrate per minute? What is the standard deviation of the counts, and therefore what is the error on your calculated value (the countrate per minute)?

(c) Estimate the probability that no counts are recorded over a random one minute interval. What is the probability that the countrate during a random one minute interval exceeds 3 counts per minute, therefore tripping the safety system?

I've done that average = 156/1440=0.108 counts/min
SD = sqrt(0.108) = 0.329

but countrate cant be <0 so not sure this makes sense. I got no idea how to do these. pls
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Joinedup
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(Original post by Feynboy)
Posting again as attatchment was lost on previous.

b)A nuclear research reactor produces radiation for neutron scattering measurements. A safety procedure shuts the reactor down if a radiation level monitoring detector measures more than 3 counts per minute. In a test, 156 counts are recorded during a random 24 hour interval. What is the corresponding average countrate per minute? What is the standard deviation of the counts, and therefore what is the error on your calculated value (the countrate per minute)?

(c) Estimate the probability that no counts are recorded over a random one minute interval. What is the probability that the countrate during a random one minute interval exceeds 3 counts per minute, therefore tripping the safety system?

I've done that average = 156/1440=0.108 counts/min
SD = sqrt(0.108) = 0.329

but countrate cant be <0 so not sure this makes sense. I got no idea how to do these. pls
It's really a stats question

I think you're sqrt-ing the wrong thing - you should be taking sqrt(n) where n is the number of counts recorded (not the rate)

the SD should be the same if you recorded 156 counts in 24 hours or 156 counts in a minute

if you want a 'better' i.e. lower SD you've got to record more counts... i.e. count for a longer time.

see p8 for an explanation https://www.cpp.edu/~pbsiegel/bio431...s/chapter2.pdf
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